1. Field of the Invention
The invention relates to the location of mobile telephone subscribers within a system and, more particularly, to a system for storing the temporary location of a mobile telephone subscriber for use in routing calls to that subscriber.
2. Description of Related Art
Radio telephone service, which has been in use for some time, originally comprised a central site capable of transmitting to a limited number of mobile or portable telephones in a large geographic area via high-powered transmitters. For clarity and convenience, the terms "mobile phone" and "mobile subscribers" will henceforth be used in this document to mean mobile or portable radio telephone stations. Because mobile phone subscriber stations employed low power transmitters, mobile phone transmissions were generally received in prior art systems by a network of satellite receivers located remotely from the central site. The satellite receivers functioned to receive and relay mobile phone transmissions to the central site for processing. In prior art systems only a limited number of radio channels were available, thus limiting the number of radio telephone conversations in an entire city to the specific number of then available channels.
In contrast, modern cellular radio telephone systems provide mobile subscribers with a comparatively large number of radio channels. These channels are effectively increased by reuse of the same radio channels within geographically separated regions within a metropolitan area. By dividing the radio coverage area of the entire cellular system into smaller coverage areas called cells and by using low power transmitters and coverage restricted receivers the call carrying capacity of the system is dramatically increased. Cellular systems of this general type are further described in U.S. Pat. Nos. 3,906,166 and 4,268,722. The limited coverage area within a cell enables the radio channels used in one cell to be reused in another cell which is geographically separated from it according to a predetermined plan, such as the exemplary four-cell pattern shown and described in U.S. Pat. No. 4,128,740. In this four-cell pattern, each cell is assigned a subset of the available radio channels and reuse of the same radio channels is repeated among groups of cells in a specific pattern throughout a metropolitan area.
A cellular system typically utilizes a pair of radio frequencies for each radio channel in each cell. Each cell is assigned at least one paging/access channel and several voice channels. The paging/access channel is employed to control the operation of the mobile phones by means of data messages transmitted to and received from each mobile phone. Functions performed in the control channel include identifying the particular cellular system to the mobile phones, instructing a mobile phone to tune to a voice channel where a conversation may take place, receiving requests for service from a mobile phone, and enabling mobile subscriber registration. The latter function refers to the process by which the mobile phones identify themselves to the system. The data message and radio channel specifications for U.S. cellular radio telephone systems are set forth in Electronic Industries Association (EIA) Standard IS-3D implemented in accordance with 47 C.F.R. 22 and Federal Communications Reports and Orders. Copies of EIA Standard IS-3D may be obtained from the Engineering Department of the Electronic Industries Association at 2001 Eye Street, N.W., Washington, D.C. 20006.
Since each cell of a cellular system may be relatively small in size, the likelihood of a mobile phone travelling out of one cell and into another cell is high. The process of switching an established call from one cell to another is known as a handoff. A cellular system may determine the need for a handoff by periodically measuring the signal strength of each active mobile phone. If the measured signal strength is below a predetermined level, the cellular system determines the availability of other channels in neighboring cells and transmits an instruction to the mobile phone to commanding it to tune to one of such available channels in another cell. Also, as the number of cellular systems increase, handoffs between different cellular systems take place in order to maintain a call in progress as a mobile phone passes from the coverage area of one cellular system to another.
Since a major goal of the mobile telephone system is the interconnection of mobile phones with the extensive public switched telephone network ("PSTN"), it is clear that successful achievement of this goal can only occur when the mobile phone is available to users of the telephone network. Because of the large number of cells in a densely populated area and the continuous movement of mobile phones from one cell to another, there arises the problem of locating a mobile phone as is moves about. If the location of the mobile is not readily accessible it cannot be interconnected with the PSTN.
A known solution to the problem of locating the mobile phone is based on the concept of mobile registration. Mobile registration is the process by which a mobile phone becomes listed as being present in the service area of one of the mobile exchanges in a mobile telephone service network. It should be recognized that one purpose of mobile registration is to permit calls to a mobile phone to be automatically delivered even though the mobile phone may be moving from place to place through a network of cellular systems. It should also be recognized that mobile phone registration according to EIA Standard IS-3D is effected by means of interactions between the cellular system and the mobile phones operating in its service area. One such interaction is called "autonomous registration" and it is controlled by the cellular system through certain information transmitted to the mobile phones. This information is in the form of an overhead message train (OMT), which is transmitted on paging channels throughout a cellular system service area, normally once each second approximately. The OMT includes a system parameter overhead message including station and registration related messages, and optionally, several other messages of which the registration identification message and the registration increment message relate to the autonomous registration process.
Registration may be enabled or disabled individually for each class of mobile phone, e.g., home or roam (explained below), by means of control bits in the system parameter overhead message. The system parameter overhead message also contains the identification number of the serving cellular system from which the mobile phone determines whether it is a "home" or a "roam" mobile phone. Each mobile phone contains, in its internal memory, an entry indicating the identity of its home cellular system and an entry indicating the cellular systems (which may be the home cellular system) in which it has most recently registered successfully. It also stores a value for the cellular system used to determine when it is scheduled to re-register in that cellular system.
In the mobile telephone systems used in North America, the United Kingdom and in other markets, twenty-one frequencies are allocated for the control channels. A two-bit digital color code (DCC) is used to differentiate control channels using the same frequency. It is thus possible to have up to 84 cells, each cell having a control channel with a unique set of frequency and DCC combinations. In densely populated areas, subscriber demand may require more than 84 cells to provide adequate mobile telephone service. In such systems, identical control channels may exist within the coverage area of a single exchange or within the coverage area of several neighboring exchanges.
As indicated, a registration or call access signal is sent over the control channel by the mobile phone and is used to identify the location of the subscriber within the mobile telephone exchange network. This signal enables a visited exchange to update the roaming subscriber's home exchange with the new location of the mobile phone. However, when identical control channels are used, such as in densely populated areas, a mobile's registration access signal may be overheard by other control channels that have identical parameters, resulting in one or more cooperating exchanges registering the subscriber as a visitor. This occurrence is known as a multiple access (usually with double access occurring) and may result in incorrect location data updating within the home exchange for the roaming subscriber. Thus, if two or more base stations linked to different Mobile Telephone Switching Offices ("MTSO"), detect a call access from a mobile phone, all of the MTSOs may each try to update the location in the home system of the mobile phone. The last MTSO to update the home system would overwrite any location data previously stored by other MTSOs, and therefore, create a high potential for the entry of erroneous location data.
A separate problem with the current methods of location updating involves what is known as a "rescan problem." The rescan problem occurs because during idle time (time during which the mobile phone is not involved in a call) the mobile phone continuously listens to the proper forward control channel. Under certain conditions it is necessary for the mobile phone to scan through all of the dedicated forward control channel frequencies ("FOCC") in order to select the signal having the best reception. The FOCC contains several pieces of information including the system identification ("SID"), a location area identification ("LOCAID"), which is defined in the EIA/TIA IS-54 specifications, and an area identification ("AID"), which is defined in the TACS, or United Kingdom, specifications. When the mobile phone detects a change in the SID, LOCAID or AID, it will send an autonomous registration access to inform the network of its location.
The problem arises when the mobile phone initiates a call, and during the call initiation it is required by the air interface to rescan and choose the best possible control channel for sending the call access, without verifying the SID, LOCAID or AID. After the call access, the mobile phone will again rescan for, and choose, a control channel. During these rescans, the mobile may be idle in MTSO A, initiate the call in MTSO B, and then return to MTSO A after the call release. All these events would occur without the mobile phone having detected a change in the system it was accessing. As a result, in this example the mobile phone does not reregister in the system of the control channel it is now tuned after the call release to update its location because it does not know it has reached a new system. The network, however, would update the mobile phone's location to MTSO B upon call initiation and thus, the mobile phone's location would incorrectly be stored as MTSO B in the home system after call release. When a call comes in for the mobile it will be sought in MTSO B, its last registered location, and be essentially lost to the system when it is not found there.
Such incorrect data within a system, as arises with the multiple access problem and the rescan problem, inefficiently uses system assets and can lead to such problems as failure of calls directed to a mobile phone. Thus, the unsatisfactorily addressed existence of multiple access and rescan problem is a shortcoming and deficiency of heretofore designed and implemented mobile communications systems.
Although no prior art solutions to the aforementioned multiple access problem are known, a number of patents contain teachings that bear some relation to the matters discussed herein. These patents are U.S. Pat. Nos. 4,833,702, 4,737,978, 4,876,738, 4,901,304 and 4,843,622. Each of these patents is discussed briefly immediately below.
U.S. Pat. No. 4,833,702 to Shitara et al. discloses a mobile telephone system having a central controller and a cordless telephone located in a local service area. The central controller transmits a periodic pre-registration polling signal to all cordless telephones which can receive it and registers the telephones from which a response signal is returned. After a cordless telephone is registered, a post-registration signal is transmitted by the telephone at periodic intervals. The central controller cancels the telephone registration if the post-registration signal is not detected for a prescribed time period.
U.S. Pat. No. 4,737,978 to Burke, et al. shows a network of cellular phone systems for use by mobile telephone units where registration for cellular system access occurs only in response to a registration ID message included in the overhead message train periodically issued by the cellular phone systems. The mobiles are paged throughout the networked coverage area starting with the coverage area in which the mobile last registered.
U.S. Pat. No. 4,876,738 to Selby shows a mobile phone which may be simultaneously registered in different service areas. The original registration in one service area may continue to be present but in an invalidated form. Thus, rather than presenting an accurate location entry, the system in Selby creates a "customized" enlarged registration area. This reduces the transmission overhead generated by the registration procedure.
U.S. Pat. No. 4,843,622 to Yotsutani et al. relates to a communication control system including a searching arrangement. Yotsutani et al.'s system uses a page sent on a control channel to locate a mobile subscriber. This system controls several cells and sends a page to a plurality of cells in a given area when it has a call to place. Upon receiving a response from a mobile in a given cell, addressing is begun to the mobile in that cell.
U.S. Pat. No. 4,901,340 to Parker, et al. shows a telephone system which allows a roaming mobile unit to direct the system to forward calls placed to the roamer's home service area to a foreign mobile telephone switching office in which the roamer is currently present. While Parker provides a Temporary Directory Number ("TDN") when in a "remote area," the TDN is for call receipt rather than location tracking purposes.
Review of each of the foregoing patents reveals no disclosure or suggestion of a solution such as that taught herein to the multiple call access problem or to the problem of rescan before and after call access.